An employee at the Creepyworld "scream park" found the 17-year-old girl, who works as an actress to scare guests at the park, Thursday night, The St. Louis Post-Dispatch reported.
Police say the girl, whose name had not been released, somehow got stuck in a noose being used as a prop. She was unconscious when they arrived and she was rushed to an area hospital.
"It's a situation that's under investigation," said Larry Kirchner, president of the park's operator, Halloween Productions Inc. "The only thing we're concerned about is our actor."
The girl was in critical condition in the hospital as of Friday night.
Creepyworld is a small park designed to look like a town hit by some sort of disaster. A massive troupe of actors scare and surprise guests who tour the park.
Police said they hadn't had any other problems at Creepyworld, but Halloween Productions, which has designed and built about 200 haunted houses worldwide, has had a number of lawsuits filed against it in recent years, including allegations that fumes and fake fog at a haunted house in St. Louis called The Darkness caused the death a 15-year-old girl in November 2010, the newspaper said.